The Role of Math5 (Atoh7) in Retinal and Optic Nerve Development and Human Disease.

Abstract

Vertebrate retinal histogenesis is controlled by both intrinsic transcriptional programs and the microenvironment. The basic helix-loop-helix (bHLH) factor Math5 (Atoh7) is required for differentiation of retinal ganglion cells (RGC), which form the optic nerve. Math5 knockout mice lack RGCs, but only 10% of Math5-expressing progenitors adopt the RGC fate, and only 55% of RGCs are lineal descendents of Math5+ cells. To define the role of Math5 in RGC development, I characterized the transcriptional anatomy of mouse Math5, and showed that it is an unspliced, single-exon gene, contrary to a recent high-profile report. I then tested the contribution of Math5-expressing cells to the earliest born cohort of mouse retinal neurons, which consist primarily of RGCs (~80%). Unexpectedly, I found that only 20-30% of this cohort expresses Math5, yet most early RGCs depend on Math5 function, suggesting a non-autonomous role for Math5-expressing cells in RGC specification. Next, I evaluated the onset of Math5 expression, and that of RGC markers Brn3b and Isl1, with respect to the terminal cell cycle. Surprisingly, these markers were expressed by neurogenic cells prior to terminal mitosis during early development (<E14), but restricted to post-mitotic cells during later stages. By retroviral clone analysis, I confirmed that early neurogenic cells often divide symmetrically, leading to paired RGC daughters. Retinal fate determination is thus not strictly synchronized to cell cycle exit. I then evaluated whether Math5 can bias terminally mitotic progenitors toward the RGC fate. I broadly over-expressed Math5 using BAC and conven-tional transgenes controlled by Crx regulatory DNA. Unexpectedly, I found that ectopic Math5 did not alter cell fate in a wild-type environment, but partially rescued RGC development in Math5 mutant retinas. Early (pioneering) RGCs are deficient in these mice, and rescue was incomplete. Transgene-derived late-born RGCs exhibited pathfinding defects and were prone to apoptosis. Finally, I evaluated the role of ATOH7 (MATH5) in human optic nerve aplasia and hypoplasia, and familial persistent hyperplastic primary vitreous (PHPV) disease. I identified a basic domain mutation (p.N46>H) in PHPV, and established causation, using biochemical and functional assays. Together, these studies provide important insights into the function of Math5 and RGC development.PHDHuman GeneticsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107270/1/lprasov_1.pd